1. Field of the Invention
The present invention relates generally to integrated circuits, and more particularly to pull-up and pull-down circuitry for input/output pads of integrated circuits.
2. Description of Related Art
Integrated circuits include input/output (I/O) pads used for receiving and transmitting signals. During operation, if an I/O pad is left “floating”, meaning that the pad is not being driven to a predictable voltage by external or internal drivers, the pad may float at a voltage level sufficient to cause input buffer circuitry connected to the pad to turn-on and operate unpredictably. This unpredictable operation increases power consumption for the device and may cause damage to the input buffer circuitry. A floating voltage on the pad may also place the input buffer circuitry in an indeterminate state, resulting in indeterminate signals propagating to the internal circuitry of the device and causing oscillations and other spurious effects.
In order to avoid such issues, integrated circuits may include pull-up or pull-down circuitry to bias the pad to a predictable voltage level when the pad is left in a floating condition. In one example, such as U.S. Pat. No. 8,400,190 (Issue Date 19 Mar. 2013, Chun Hsiung Hung et al.), a pad can be pre-charged to either a high reference voltage or a low reference voltage, as manufactured. However, a pad of an integrated circuit may need to be coupled to a pull-up circuitry for one application, but coupled to a pull-down circuitry for another application, depending on requirements of a system that includes the integrated circuit.
It is therefore desirable to provide improved pull-up and/or pull-down circuitry for pads of integrated circuits that allow setting individual pads to predictable voltage levels.